The starting material for the production of soap bars or billets is a mixture containing surfactants, other functional ingredients and water at appropriate proportions. Depending upon the composition of this mixture, its rheological and processing characteristics vary a great deal.
Generally, processing/finishing of such a mixture involves various process steps such as homogenisation, shear working, and forming into a required shape.
One of the devices very commonly employed to carry out one or more of the above operations is a plodder.
The function of a simple plodder is to form the mixture into bars or billets of required cross-sections which may subsequently be cut into smaller bars or stamped into tablets of required shape by suitable other means.
The function of a refiner plodder is to clean the mixture free of gritty particles or impurities and additionally homogenise/shear work the same to achieve the required degree of homogeneity or phase structure. The plodders may also be used to convert loose aggregates/chips/flakes into pellets or noodles for intermediate storage or for feeding subsequent process operations.
The heart of a plodder assembly is a screw extruder. The simplest plodder has an extruder with a single screw. The feed stock, either in homogenised and worked form or in the form of pellets, noodles, crimpled chips or (recycled) bars fed through the hopper enters the extruder barrel and fills the annular space between the extruder worm (screw) and the barrel. The barrel is stationary and the worm rotates inside the barrel. Frictional/viscous drag forces act on the material, both at the barrel as well as at the worm surfaces. The resultant force is responsible for the forward transportation of the processed mass like a rotating nut on a stationary screw towards the discharge end. At the discharge end, the extruder may have a perforated plate, through which, the processed mass is forced. This is generally known as the `noodle` plate. The processed mass emerges in the form of rods/ribbons/sheets from the perforated plate.
If the objective is to produce chopped noodles or pellets for intermediate storage or to serve as a feed for subsequent process step then a suitable cutter is provided at the discharge end to chop the extrudates into smaller pieces. If the objective is to enhance homogenisation or shear working or to filter out gritty particles, then it is advantageous to fit a wire gauze in front of the perforated plate but it tends to reduce throughput. If the objective is to form the mixed mass into billets or bars, then a cone and die/eye plate are provided at the discharge end of the extruder along with or without the perforated plate. The extruder forces the mixed mass through these end fittings to produce the billets or bars. Designs of perforated plates, cones and die/eye plates vary considerably from application to application.
Machines called duplex or twin worm plodders have two worms (or screws) which are parallel, non-intermeshing and mounted tangentially with respect to each other within a barrel. The worms may be co-rotating but usually they are counter-rotating. Intermeshed and co-rotating twin screw extruders are also known for processing of soap/detergent mass. In the case of both non-intermeshed duplex plodders as well as the intermeshed co-rotating twin screw extruders, drag forces similar to those encountered in single worm plodders act on the processed mass and push the same in the forward direction.
Screw extrusion is apparently a simple operation, but the results in terms of quality of product, throughput rate, specific energy consumption, etc. can be influenced by a number of factors in a rather complex way. Generally, plodding is affected by soap factors and by machine factors. It is important to balance the various factors so as to achieve the best results. As the processed material moves forward, it gets heated up as a result of frictional and shear heat generation. Considerable structural breakdown may also take place in the case of some formulations. In certain instances, the heat generated raises the temperature of the processed mass above desirable limits thereby adversely affecting certain properties of the soap. The following are a few disadvantages of the hitherto known soap finishing processes employing single worm plodders, duplex plodders and intermeshed co-rotating extruders:
1.The throughput rate is very sensitive to the resistance offered by wire mesh screens, perforated plates, cones and dies.
2. The processed material being transported may be subjected to excessive shear which may result in rheological damage and temperature rise. These may lead to deterioration of user properties as well as softer extruded forms which may cause complications in downstream processing. The excessive and wasteful shear dissipation may also lead to lower energy efficiency.
3. It is necessary to employ a refrigerated coolant with less than 15.degree. C. preferably less than 10.degree. C. temperature for cooling the barrel to achieve acceptable temperature control of product and pumping characteristics. This requirement results in an additional investment in a refrigeration plant and additional operating expenditure.
4. The transport characteristics of the conventional extruders are very sensitive to the rheology of the processed material. The rheology of soap/detergent bars in turn is very sensitive to their formulations. Therefore there are several restrictions on formulations that can be processed satisfactorily. These restrictions severely limit manufacture of soap/detergent bars with improved/novel functional benefits such as skin emoliency, moisturisation, etc. which can be achieved through formulation changes involving use of alternative detergent actives, such as alkyl isethionates, functional ingredients, such as skin benefit agents, for example fatty acids, mineral and paraffin oils and silicones.
In the history of plodded soap bars, which is larger than half a century, equipment manufacturers as well as manufacturers of soap bars have investigated and developed plodder worms with different design features and parameters none of which are able to adequately reduce the above disadvantages.
An object of the present invention is to minimise, and preferably avoid, the above disadvantages.